This invention relates to a process for concentrating aqueous fluoropolymer dispersions, such as dispersions of polytetrafluoroethylene.
Concentrated fluoropolymer dispersions of higher weight percent than those obtained directly from polymerization are preferred in uses such as the coating of glass fabric, because fewer coatings are required to build the necessary thickness of fluoropolymer. Concentrated aqueous fluoropolymer dispersions are also preferred in uses such as the coating of metals and circuit boards. Such dispersions are also used to make films.
Various techniques are known for providing concentrated fluoropolymer dispersion, but all suffer from one or more disadvantages. Concentration of polytetrafluoroethylene dispersions to greater than 25 wt % is disclosed in U.S. Pat. No. 2,478,229. The action of naturally derived substances such as ammonium alginate, locust bean gum, gum tragacanth, and derivatives of cellulose to concentrate dispersions of perfluorohaloolefin polymers is disclosed in U.S. Pat. No. 2,878,196 to Buffington and U.S. Pat. No. 3,778,391 to Morris et al.
The concentration method which enjoys the widest commercial use is disclosed in U.S. Pat. Nos. 3,037,953; 3,301,807 and 3,704,272. The technique involves the addition of non-ionic surfactants such as ethoxylated alkyl phenols or ethoxylated aliphatic alcohols to a fluoropolymer dispersion obtained directly from polymerization. The resulting mixture is heated above its cloud point, which causes it to separate into upper and lower phases. The upper phase contains a significant portion of the non-ionic surfactant, making environmentally acceptable disposal of this phase expensive. The large amount of non-ionic surfactant needed, typically 3 wt % or greater based on polymer solids, is expensive. For some applications, the use of lower amounts of non-ionic surfactant would provide benefits to the end product, such as a more desirable color or ease of manufacture.